Promoted iron oxide catalysts have been known for many years as dehydrogenation catalysts. They are especially useful in the manufacture of styrene by the dehydrogenation of ethylbenzene. Most of the catalysts now in commercial use employ minor amounts of promoters, e.g. salts or oxides of chromium, manganese, bismuth, tungsten, or molybdenum, with chromium being the preferred minor component, together with compound of potassium, e.g. potassium oxide or carbonate. The last component gives the catalyst a self-regenerative property enabling its use for long periods of time without significant loss in activity. More recent improvements include the incorporation of minor amounts of vanadium, cerium, and of modifiers (such as carbon black or graphite and methyl cellulose) which affect the pore structure of the catalysts. None of these improvements have dealt with the physical integrity of the catalysts. Improved stability to moisture is desirable while maintaining high activity and high yield.
Catalyst life of dehydrogenation catalysts is often dictated by the pressure drop across a reactor, the increase of which lowers both the yield and conversion to the desired vinyl aromatic. For this reacon, the physical integrity of the catalyst is of major importance.
In recent years catalysts with higher amounts of potassium have been used, e.g. 20% or more, up to about 48% of potassium calculated as the oxide Thus, in U.S. Pat. No. 4,503,163 catalysts are disclosed which contain 13-48% and preferably 27-41% by weight of a potassium promoter compound, calculated as potassium oxide. Such catalysts are self regenerative catalysts which perform well at lower steam to oil ratios, e.g. ratios of &lt;2/1. The economic advantages of using less steam are obvious. Associated with the higher amounts of potassium used in such catalysts has been an increase in physical degradation due to moisture and "wet steam" during start-up conditions and plant upsets. This physical degradation can cause increased pressure drop due to coking, plugging, and reduction of the void volume in the catalyst bed.
It would be desirable if a catalyst could be prepared which had both high activity and resistance to moisture. A method has now been discovered which will provide such a catalyst.